format long;
clear;
pkg load symbolic;
syms R1  R2  C1 C2  R3  R4  S  Ui
a=[-1/R1-C1*S-1/R2,1/R2,C1*S;1/R2,-1/R2-C2*S,0;0,-1/R3-1/R4,1/R4];
b=[-Ui/R1;0;0];
zs=inv(a)*b;
simplify(zs)

